dont do what i do and keep it bare bottom. go for sand and fish that swim on the top/middle surfaces. arowana would be the perfect tank mate. thats what i kept mine with.
if you kept rays then you will know that they **** like humans. with bare bottom you can syphon it out beforen it starts rotting away and play havoc with the water chemistry. but i recommend fine sand (ironic i know) cos its there home.
and as regards to tank size. if you have a small ray the bare minimum i recommend is a 4x2x2. . . that would see set you up for a year. then plan for an upgrade size.
but, ray purists would say im wrong . . .
teacups should work well in a tamk your size. they dont grow too big and slow growers also. any none aggressive tankmates like bala sharks,torpedo barbs, would look great with them.
ok well i think a 120 is better than the 30g with jagged gravel.. its a great store just 1 bad decision. so how would i syphon sand ive never had sand... and i really dont like bare bottom but if i have too... and the 30 is pretty clean i didnt see to much poop and they just had a power head in there...
and the tea cup thing im new to stingrays and i heard theyre hard to keep so im sure i could if i did enough reasearch but i just want to have some expierience before i try to keep a tea cup sting ray